Allium white rot

What you are looking at

The older leaves of an onion or a leek yellow and die back from the tip while the plant is still young. It wilts, and when you take hold of it, it lifts out of the ground with no resistance at all, because there are no roots left to hold it.

Turn the base over. There is a dense white fluffy growth over the base plate and where the roots were, and embedded in it are small hard black grains about the size of poppy seeds.

Those grains are the diagnosis and the problem in one. Nothing else in a British vegetable garden produces white fungal growth studded with black grains at the base of an allium.

Garlic and shallots suffer worst, onions next, and leeks usually least, though none of them escapes.

Twenty years in the soil, waiting

The black grains are sclerotia, resting bodies with a hard coat, and they can sit in soil for fifteen to twenty years without a host.

What makes this particularly hard to shift is how they wake up. A sclerotium stays dormant until it detects the sulphur compounds that allium roots release into the soil, and only then does it germinate and grow the short distance to the root. It is a lock that only onions, garlic, leeks, shallots and chives carry the key to.

That is why rotation, which handles a great many soil problems, does not handle this one. Nothing you grow in between triggers the sclerotia, so they wait. A three-year rotation is meaningless here and even an eight-year one is a delay rather than a cure.

It is also why there is no cure and no chemical. Nothing available to a gardener or to a commercial grower removes sclerotia from soil, and no resistant onion exists. Commercial growers sometimes use garlic oil to bring the sclerotia out of dormancy when there is no crop for them to infect, so they germinate and starve, and that technique is not sold at a garden centre scale.

What works instead

Grow alliums somewhere else. Not another bed on the same plot, since the sclerotia will get there eventually on a boot, but in containers or a raised bed of bought compost standing clear of the infected ground.

Onions, garlic and leeks grow perfectly well in a deep tub. Allotment holders who lost their onion bed a decade ago are still eating home-grown garlic on exactly this basis.

Then keep the infection where it is. Clean boots, a dedicated fork, no gifted plants with soil on the roots, and every affected plant dug out with the soil around it and binned rather than composted. That is how the other half of the plot stays usable, and it is how you avoid giving it to a neighbour.

What to do about it

Move allium growing into containers or raised beds with fresh compost.

Lift every affected plant with a spadeful of the soil around it and bin or burn the lot.

Never compost anything from an affected plant.

Scrape and wash boots and tools before moving off the bed.

Refuse onion and leek plants raised in someone else’s soil.

Rotate as far as your plot allows, while expecting it to delay rather than fix the problem.

Pull wild garlic, three-cornered leek and self-sown onions from around the plot.

Use the infected bed for everything else, which is most of what you grow.

What it gets mistaken for

Ordinary root rot follows waterlogging and leaves the roots dark, soft and shrunken with no fungal growth you could point to and no black grains.

Downy mildew greys and collapses onion leaves from halfway up and leaves the base of the plant sound.

Leek rust marks the leaves with bright orange pustules that come off on a finger.

Onion neck rot shows up in store rather than in the ground, softening the neck of a lifted bulb with a grey fuzzy growth.

Clubroot is the closest thing to this on the plot, doing the same job on brassicas with the same twenty-year persistence. Neither has a cure and both have the same answer, which is to grow the crop in clean compost instead.